1. Field of the Invention
This invention relates generally to implanted medical devices and, more particularly, relates to a physiological waveform morphology discrimination method and apparatus for use in characterizing the origin of cardiac depolarizations and adjusting the operation of the medical device accordingly.
2. Description of Prior Art
Early automatic tachycardia detection systems for automatic implantable cardioverter/defibrillators relied upon the presence or absence of electrical and mechanical heart activity (such as intramyocardial pressure, blood pressure, impedance, stroke volume or heart movement) and/or the rate of the electrocardiogram. For example, the 1961 pamphlet by Dr. Fred Zacouto, Paris, France, entitled. "Traitement D'Urgence des Differents Type de Syncopes Cardiaques du Syndrome de Morgangni-Adams Stokes" (National Library of Medicine) describes an automatic pacemaker and defibrillator responsive to the presence or absence of the patient's blood pressure in conjunction with the rate of the patient's electrocardiogram. Later detection algorithms proposed by Satinsky, "Heart Monitor Automatically Activates Defibrillator," Medical Tribune, 9, No. 9I:3, Nov. 11, 1968, and Schuder et al "Experimental Ventricular Defibrillation with an Automatic and Completely Implanted System," Transactions American Society for Artificial Internal Organs, 16:207, 1970, automatically detected and triggered defibrillation when the amplitude of the R-wave of the electrocardiogram fell below a predetermined threshold over a predetermined period of time. The initial system proposed by Mirowski et al in U.S. Pat. No. Re. 27,757, which similarly relied upon the decrease in the amplitude of a pulsatile right ventricular pressure signal below a threshold over a predetermined period of time, was abandoned by Mirowski et al in favor of the rate and/or probability density function morphology discrimination as described in Mower et al, "Automatic Implantable Cardioverter Defibrillator Structural Characteristics," PACE, Vol. 7, November December 1984, Part 11, pp. 1331-1334. Others have suggested the use of high rate plus acceleration of rate or "onset" (U.S. Pat. No. 4,384,585) with sustained high rate and rate stability (U.S. Pat. No. 4,523,595).
Very generally, the systems that depend upon the aforementioned criteria are capable of discriminating tachyarrhythmia in greater or lesser degree from normal heart rhythm but have difficulty discriminating sinus or other supraventricular tachycardias from malignant, pathologic ventricular tachycardias, resulting in the delivery of inappropriate cardiac electrical stimulation therapies.
A stated in the article "Automatic Tachycardia Recognition" by R. Arzbaecher et al (PACE, May-June 1984, pp. 541-547), antitachycardia pacemakers that were undergoing clinical studies prior to the publication of that article detected tachycardia by sensing a high rate in the chamber to be paced. The specific criteria to be met before pace termination was to be attempted involved a comparison of the detected rate to a preset threshold, such as 150 beats per minute (400 millisecond cycle length) for a pre-selected number of beats. As stated above, other researchers had suggested the rate of change of rate or suddenness of onset, rate stability and sustained high rate a additional criteria to distinguish sinus tachycardias from malignant tachycardias. Arzbaecher et al proposed in their article an algorithm implemented in a microprocessor based implantable device employing both atrial and ventricular rate detection via separate bipolar leads in order to detect the AA and VA, or VV and AV intervals (or "cycle lengths") against threshold intervals in order to distinguish pace-terminable and nonpace-terminable tachycardias. Arzbaecher et al introduced the concept of employing a single atrial extra stimulus to distinguish sinus tachycardia from 1:1 paroxysmal tachycardia in order to determine whether a ventricular response would be elicited. An atrial extra stimulus was delivered in late diastole (80 milliseconds premature), and the ventricular response, if appearing early as well, indicated that the patient was in sinus rhythm. However, in pace-terminable tachycardias, such as AV reentrant and ventricular with VA conduction tachycardia, the ventricular response would not occur early (indicating that the atrial and ventricular rhythms were disassociated) and the ventricular rhythm would be unperturbed.
Other proposals for employing atrial and ventricular detection and interval comparison are set forth in The Third Decade of Cardiac Pacing: Advances in Technology in Clinical Applications, Part III, Chapter 1, "Necessity of Signal Processing in Tachycardia Detection" by Furman et al (edited by S. Barold and J. Mugica, Future Publications, 1982, pages 265-274) and in the Lehmann U.S. Pat. No. 4,860,749. In these cases also, atrial and ventricular rates or intervals are compared to one another in order to distinguish sinus and pathological tachycardias.
Another approach to the detection of and discrimination between pathologic and sinus or normal tachycardias involves the comparison of current electrogram morphologies to a stored library of morphologies in the manner shown for example in the U.S Pat. No. 4,523,595. In such systems, the suspect electrograms are continuously digitized and compared against the reference digitized electrograms to find the closest fit and diagnose the suspect rhythm.
The aforementioned discussion reflects the development in the art of the detection and discrimination of spontaneously occurring atrial and ventricular tachycardias. In the field of dual chamber atrial synchronous heart pacemakers, such as multiprogrammable DDD pacemakers, the generation of ventricular stimulation pulses ensues after an AV delay time following the detection of an atrial or P-wave signal. The ventricular stimulation rate varies within a relatively wide range from a programmable lower rate, such as 50-75 beats per minute, to a programmable upper rate, such as 100-140 beats per minute. The ventricular stimulation rate may track the sensed atrial P-wave rate up to the upper rate limit whereupon synchronization may be lost periodically, the pacemaker exhibiting a pseudo-Wenckebach behavior, as described in Adams U.S. Pat. No. 4,059,116, for example.
If retrograde conduction exists in the patient's heart, each ventricular stimulus may evoke a depolarization that is conducted back to the atrium, causing it to contract. The atrial sense amplifier may respond to the corresponding P-wave, and in turn, trigger the generation of a ventricular stimulus at or near the upper rate limit of the pacemaker. This behavior of the pacemaker is referred to as "pacemaker-mediated tachycardia" or PMT. In order to prevent PMT, most DDD pacemakers include a programmable post ventricular atrial refractory period that the physician may extend to cause the atrial sense amplifier to ignore the retrograde induced P-wave. However, lengthening the refractory time, in effect, reduces the upper rate limit and is, therefore, disadvantageous to the patient. Another proposal has been to provide timing windows for detecting the closely-coupled retrograde P-wave and to switch the paging mode of operation to a single chamber mode, such as VVI pacing. In U.S. Pat. No. 4,802,483, circuitry is provided to improve the transition between VVI pacing and DDD pacing to avoid synchronization to atrial activity triggered by retrograde transition. In DDD pacemakers, it remains desirable to provide a reliable method and apparatus for distinguishing atrial activity triggered by retrograde conduction from normal physiologic atrial activity and to provide an appropriate response mode.